In the past, the practice had been to produce a hot stamped part by a hot stamping process. At that time, a blank comprised of a plurality of steel sheets joined at the surfaces is supplied to the hot stamping process. Due to this, the strength and rigidity of the hot stamped part are raised. In this case, one or more steel sheets are joined at a region of the main steel sheet requiring larger strength and rigidity than other regions. As applications for such hot stamped parts using such blanks, for example, there are component parts of monocoque bodies (unit construction bodies) such as front side members or center pillars etc.
To join such a main steel sheet and a reinforcing steel sheet, there is the technique of using brazing.
PLT 1 discloses the art of placing a brazing filler metal (hard solder, same below) between the surfaces of the main steel sheet and reinforcing steel sheet and welding them to provisionally fasten the steel sheets and brazing filler metal, then heating the steel sheets to a temperature higher than the shaping temperature of the steel sheets and hot press-forming them and then cooling the steel sheets in the die after shaping so as to cause the brazing filler metal to solidify.
PLT 2 discloses the art of clamping a brazing filler metal with a solidus temperature of 1050° C. or less and a liquidus temperature of 700° C. or more between the surfaces of a main steel sheet and reinforcing steel sheet, placing the blank in a heating furnace to heat it to 900° C. to 1050° C., then quenching and press-forming (hot stamping) it.
PLT 3 discloses the art of coating a sheet surface with a brazing filler metal, heating the coated main steel sheet for the quenching and making the brazing filler metal melt, joining the surface of the main steel sheet and the surface of the reinforcing steel sheet through this melted brazing filler metal, then quenching and press-forming (hot stamping) the same.